The present invention relates to pulse coded inverters for utility load management systems and more particularly to control apparatus for pulse coding static inverters for utility load management systems.
In electrical utility systems various types of load management control have been employed. With load management control, the utilities have the capability of interrupting service to non-critical loads during times of peak power demand. In the past such control has been exercised by timing devices, signals from telephone lines, radio signals and ripple control.
The present invention is directed to load management utilizing ripple control.
In a ripple control system, an audio frequency signal is impressed directly onto the utility's power lines and is transmitted along with and superimposed upon the high voltage being transmitted. The signal is pulse coded to provide individual channels of information. Thus the signal gives two levels of selection and identification for signal security, i.e. the magnitude of the audio frequency and the particular code of the pulses. Hence a wide selection of channels is available through pulse coding, and adjacent coupled power systems are protected from interference by audio tone diversity. This system allows the utility full control over its communication network, and permits it to transmit over substantial distances.
Ripple control transmitters have, until recently, been electromechanical in nature. These usually consisted of a motor/alternator set operating through thyristor static switches to apply the pulse coding. The alternator, however, is specific to a given audio frequency and must be specially constructed for each application.
In the present invention the high power audio frequency pulses are produced by a solid state static inverter and superimposed on the network by means of a suitable coupling network. The static audio-frequency transmitters are better suited to this purpose than the previously used rotary converters. They can be installed without any foundations in closed operating rooms provided for electrical equipment. They operate with high frequency accuracy. The frequency is adjustable and is independent of the network frequency. They are instantly ready for operation since there is no long starting time. One important advantage of the static transmitter over the rotary converter is the totally synchronous injection of the audio-frequency pulse code in the case of several separate transmitters in meshed networks. Therefore, there will be no beat in any connected network sections which may cancel the signal. Synchronous and equal-phase injection can be insured by means of a central control.
Thus with systems of the present invention receivers are located at a customer's line and are adapted to recognize both a selected low frequency pulse code and a relatively high frequency signal. The high frequency signal is in the audio range and can be varied from 300 to 1500 Hz. The pulse code will be at a much lower frequency. Thus with two distinct signal components required, the possibility of error at the receiver is reduced. Loads of different character can be controlled by different pulse codes permitting the utility a wide range of selectivity in its load control.
Since the control signals must be injected onto the high voltage transmission line, there will be induced backfeed currents and these must be handled by the static inverter. The apparatus of the present invention handles the backfeed current during all modes of operation of the ripple control system while preventing buildup of excessive voltage from the transmission line.
The control signals are injected onto the transmission line via an isolation transformer. It has been found that core saturation can occur from unidirectional current and/or phase imbalance; if this occurs, the impedance of the transformer can be reduced possibly resulting in currents of high magnitude and potential damage to the associated apparatus. Such imbalance has been precluded in the present sysem by providing that the signal current through the isolation transformer will be equally balanced between positive and negative half cycles as well as from phase to phase whereby core saturation will be precluded.
Therefore, it is an object of the present invention to provide an improved ripple control system for electrical utility load management.
It is another object of the present invention to provide an improved static inverter and control logic therefor for a ripple control system.